Crystal Handedness and Spin Helix Chirality in Fe1-xCoxSi

We show, with the help of polarized neutrons, that the cubic magnets Fe1-xCoxSi with Dzyaloshinskii-Moriya interaction can be switched between left (for x=0.1, 0.15) and right (for x=0.2, 0.25, 0.3, 0.5) chiral states of the spin helix. The absolute structure was evaluated using x-ray diffraction. The crystals are shown to be enantiopure and the structural chirality changes from right handed for x < 0.2 to left handed for x > 0.2. These compounds are compared with the etalon sample of MnSi which is identified as having the left-handed chirality both in the magnetic and crystallographic sense.